U.S. patent application number 12/735206 was filed with the patent office on 2010-10-21 for arrangement for the potential-free measurement of currents.
This patent application is currently assigned to Sensitec GmbH. Invention is credited to Jochen Schmitt.
Application Number | 20100264905 12/735206 |
Document ID | / |
Family ID | 40622271 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100264905 |
Kind Code |
A1 |
Schmitt; Jochen |
October 21, 2010 |
ARRANGEMENT FOR THE POTENTIAL-FREE MEASUREMENT OF CURRENTS
Abstract
The invention relates to an arrangement for the potential-free
measurement of current flowing in two primary conductors (13, 14)
arranged in parallel in opposite directions of each other, wherein
the magnetic differential field is detected by means of a
differential field sensor (12). In order to reduce measuring
errors, the primary conductors (13, 14) and the differential field
sensor (12) are disposed between two metal parts that are
configured as shield plates (1, 2) made of a highly permeable
material, each having a web (3 or 6). The webs (3, 6) extend on
both sides of the arrangement of the primary conductors (13, 14)
substantially in the direction thereof parallel to the arrangement.
At least one of the shield plates (1, 2) is configured in a U-shape
in cross-sectional planes, having one pair of limbs (4, 5; 7, 8)
each, which extend at a right angle in a longitudinal direction to
the U-shaped cross-sectional planes. The two shield plates (1, 2)
are disposed relative to each other such that at least one limp
pair (4, 5; 7, 8) of one of the two shield plates (1,2) is aligned
toward the other of the two shield plates (1, 2) on one of the two
limbs (4, 5; 7, 8) while leaving an air gap (9, 10).
Inventors: |
Schmitt; Jochen;
(Biedenkopf, DE) |
Correspondence
Address: |
Quinn Emanuel Urquhart & Sullivan, LLP
865 S. FIGUEROA STREET, 10TH FLOOR
LOS ANGELES
CA
90017
US
|
Assignee: |
Sensitec GmbH
|
Family ID: |
40622271 |
Appl. No.: |
12/735206 |
Filed: |
December 18, 2008 |
PCT Filed: |
December 18, 2008 |
PCT NO: |
PCT/EP2008/010833 |
371 Date: |
June 21, 2010 |
Current U.S.
Class: |
324/126 |
Current CPC
Class: |
G01R 33/09 20130101;
G01R 15/205 20130101; G01R 15/207 20130101; G01R 33/022
20130101 |
Class at
Publication: |
324/126 |
International
Class: |
G01R 1/20 20060101
G01R001/20 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2007 |
DE |
10 2007 062 633.0 |
Claims
1. Arrangement for the potential-free measurement of currents
flowing in two primary conductors (13, 14; 25, 26; 25', 26'; 34,
35) arranged substantially in parallel, in opposite directions of
each other, by detecting the magnetic differential field by means
of a differential field sensor (12, 24, 24', 37), wherein for
reducing measuring errors, said primary conductors (13, 14; 25, 26;
25', 26'; 34, 35) and said (12, 24, 24', 37) are disposed between
two metal parts, characterized in that the two metal parts are
configurated as shield plates (1, 2; 15, 16; 15', 16'; 27-30) made
of highly permeable material each having a web (3 or 6,
respectively), that said webs (3, 6) extend on both sides of the
arrangement of the primary conductors (13, 14; 25, 26; 25', 26';
34, 35) substantially in the direction thereof parallel to the
arrangement of the primary conductors (13, 14; 25, 26; 25', 26';
34, 35), that at least one of said shield plates (1, 2; 15, 16;
15', 16'; 27-30) is configurated in a U-shape in cross-sectional
planes having a pair of limbs (4, 5; 7, 8; 17-20; 19', 20') each of
which extend at a right angle in a longitudinal direction to the
U-shaped cross-sectional planes, that said two shield plates (1, 2;
15, 16; 15', 16'; 27-30) are disposed relative to each other such
that the at least one limb pair (4, 5; 7, 8; 17-20; 19', 20') of
one of said two shield plates (1, 2; 15, 16; 15', 16'; 27-30) is
aligned towards the other of said two shield plates (1, 2; 15, 16;
15', 16', 27-30 while leaving one air gap each (9, 10; 21, 22; 21',
22'; 39, 40) on one of said two limbs (4, 5; 7, 8; 17-20; 19',
20'), and that said primary conductors (13, 14; 25, 26; 25', 26';
34, 35) extend in the longitudinal direction of said limbs (4, 5;
7, 8; 17-20; 19', 20').
2. Arrangement according to claim 1, characterized in that said two
shield plates (1, 2; 15, 16; 27-20) are configurated with one limb
pair (4, 5; 7, 8; 17-20) each between which one of said webs (3,6)
each is disposed, and that said shield plates (1, 2; 15, 16; 27-30)
are arranged relative to each other such that said two limb pairs
(4, 5; 7, 8; 17-20) of said shield plates (1, 2; 15, 16; 27-30) are
aligned toward each other while leaving one air gap (9, 10; 21, 22;
39, 40) each between two of said limbs (4, 5; 7, 8; 17-20).
3. Arrangement according to claim 1 or 2, characterized in that
said primary conductors (13, 14; 25, 26; 25', 26'; 34, 35) are
substantially arranged between said limbs (4, 5; 7, 8; 17-20; 19'
20') of one of said two shield plates (1, 2; 15, 16; 15', 16';
27-30).
4. Arrangement according to claim 1 or 2, characterized in that
said effective sensor plane (11, 23, 36) is disposed outside of
said limbs (7, 8; 19-20) of said shield plate (2, 16, 28, 30)
between which limbs (7, 8; 19-20) said primary conductors (13, 14,
25, 26, 35, 36) are arranged.
5. Arrangement according to claims 1-4, characterized in that said
limbs (4, 5; 7, 8; 17-20) of said two shield plates (1, 2, 15, 16,
27-30) are equally long and that said two shield plates (1, 2; 15,
16; 27-30) are symmetrically disposed relative to an effective
sensor plane (11, 23, 36).
6. Arrangement according to claims 1-4, characterized in that said
limbs (17, 18; 19, 20) of one of said shield plates (15, 16) each
are equally long, that said limb pairs (4, 5; 19, 20) of said
shield plates (15, 16) have different lengths, and that said
primary conductors (25, 26) are substantially arranged between said
longer limbs (19, 20) of one of said two shield plates (15,
16).
7. Arrangement according to one of claims 1-6, characterized in
that said shield plates (1, 2; 15, 16; 15', 16'; 27-30) extend in
the longitudinal direction of the limbs thereof (4, 5; 7; 8; 17-20;
19' 20') substantially above said differential field sensor (12,
24, 24', 36).
8. Arrangement according to one of the foregoing claims,
characterized in that said two parallel primary conductors (13, 14;
25, 26; 25', 26', 34, 35) constitute the limbs of a substantially
U-shaped conductor unit.
9. Arrangement according to one of the claims 1-7, characterized in
that said two parallel primary conductors (13, 14; 25, 26; 25' 26',
35, 36) are the to and fro conductor of an electric consuming
device.
10. Arrangement according to claim 8, characterized in that said
shield plates (1, 2; 15, 16; 15', 16'; 27-30) extend in the
longitudinal direction of the limbs thereof (4, 5, 7, 17-20)
substantially over the length of said limbs (4, 5; 7, 8; 17-20;
19', 20') of said U-shaped conductor unit in the direction of the
current.
11. Arrangement according to one of the foregoing claims,
characterized in that said shield plates (1, 2; 15, 16; 15', 16';
27-30) consist of mumetal.
12. Arrangement according to one of claims 1-10, characterized in
that said shield plates are electric sheets.
13. Arrangement according to one of the foregoing claims,
characterized in that for the establishment of the magnetic
differential field in the effective sensor plane (11, 23, 36) at
least one soft magnetic insert part (31-33) is used which is
arranged with the said two shield plates (1, 2; 15, 16; 27-30).
14. Arrangement according to claim 13, characterized in that the at
least one insert part (31-33) is disposed between said effective
sensor plane (11, 23, 36) and said primary conductors (13, 14; 25,
26; 34, 35).
15. Arrangement according to claim 14, characterized in that
between each of said primary conductors (34, 35) and said effective
sensor plane (36) one insert part each (31, or 32, respectively) is
disposed.
16. Arrangement according to claim 14, characterized in that
between said two primary conductors (34, 35) and the effective
sensor plane (36) only one insert part (33) is disposed
symmetrically to a middle plane (38) between said primary
conductors (34, 35).
17. Arrangement according to claim 14, characterized in that the at
least one insert part is disposed between said active sensor plane
and the nearest neighboring shield plate.
18. Arrangement according to one of the foregoing claims,
characterized in that said air gaps (39-42) are adjustable on or
between, respectively, said limbs of said shield plates (27, 28;
29, 30).
Description
[0001] The invention relates to an arrangement for the
potential-free measurement of currents according to the preamble of
claim 1.
[0002] Prior art arrangements of this type measure currents by
detecting a magnetic differential field, or a magnetic field
gradient, respectively, generated by a current in a substantially
U-shaped conductor unit the limbs of which may be referred to as
primary conductors (WO 00/11482, DE 4300605 C2, U.S. Pat. No.
5,548,208).
[0003] The magnetic differential field, or the magnetic field
gradient, respectively, penetrates a differential field sensor, or
a gradiometer, respectively, which generates a signal voltage
corresponding to the magnetic differential field, or the magnetic
field gradient, respectively, without being galvanically connected
with the primary circuit. Thereby, a measuring system is desired
without any elaborate magnetic flow conduction while the influence
of magnetic interference fields is nonetheless minimized.
[0004] Sensors based on magneto resistive effects such as the
anisotropic magneto resistive effect (AMR) or the gigantic magneto
resistive effect (GMR) may be used in particular as suitable
differential field sensors. Such sensor systems on the basis of the
colossal magneto resistive effect (CMR) or of the tunnel magneto
resistive effect (TMR) are conceivable as well.
[0005] In the case of arrangements for the potential-free
measurements of currents of the kind referred to in the beginning,
it is the task as mentioned before to reduce a sensibility
vis-a-vis external magnetic interference fields which may disturb
the detection of the magnetic differential field, or the field
gradient, respectively, relevant for the measurement. The external
magnetic fields may, inter alia, be caused by eddy currents which,
on their part, are induced in adjoining metal parts by the magnetic
field of the current to be measured, referred to as primary
current. In order to reduce the frequency-depending measuring
errors caused thereby, it has already been known in an arrangement
of the kind referred to in the beginning to adjust the
cross-sectional area and the form as well as the material selection
of the primary conductors and their distance from particularly
shaped neighboring metal parts, if any, and the magnetic field
sensitive measuring devices, particularly sensors, so that the
influences of different electro-dynamic effects, particularly of
the induction of eddy currents in the neighboring metal parts,
largely compensate each other (DE 19819470 A1, particularly FIG.
4). In detail, an arrangement of this kind is realized by a
conductor unit made in U-shape on which a dielectric plate is
fastened which carries a magnetic field measuring device and causes
a galvanic separation. Above and below this arrangement, the
above-referenced neighboring metal parts, particularly plates, are
disposed, spaced in predetermined order.
[0006] It is the object of the present invention to further reduce,
in an arrangement for potential-free measurement of currents of the
kind referred to in the beginning, the influence of external
interference fields on the detection of the magnetic differential
field, or the field gradient, respectively, of two substantially
parallel primary conductors in a technically less sophisticated way
and to obtain, by the means to be used, an additional technical
benefit.
[0007] This task is solved by the features referred to in claim
1.
[0008] By means of the two metal parts shaped, according to the
invention, as shield plates made of highly permeable material at
least one of which plates being configured substantially U-shaped
in cross-sectional planes, having one pair of limbs each which
extend at a right angle in a longitudinal direction to the U-shaped
cross-sectional planes wherein the two shield plates are disposed
relative to each other such that the at least one limb pair of one
of the two shield plates is aligned toward the other of the two
shield plates while leaving an air gap each on one of the two limbs
and wherein the primary conductors extend in the longitudinal
direction of the limb, a very good shielding of the differential
field sensor which is disposed, as are the primary conductors,
within the space enclosed by the shield plates, is obtained against
the external magnetic interference fields. Nonetheless, the
magnetic field gradients originating from the primary conductors
detected by the differential field sensor are not too much
attenuated since the two shield plates do not form a completely
closed magnetic circle about the primary conductors. The
dimensioning of the shield plates which are formed and disposed
according to the features of the claim may be obtained by simple
trials or simulation calculations of the field configuration,
particularly at the site of the differential field sensors.
[0009] As an additional benefit, in this connection, the measuring
sensibility, or the measuring range, respectively, of the
arrangement for the potential-free measurement of currents can be
set, namely by variation of the air gap and/or the dimensions,
particularly of the thickness of the shield plates.
[0010] In an effective variant of the arrangement of the invention,
the two shield plates are shaped as having one pair of limbs each
between which one of the webs each is disposed, and these two
shield plates are arranged relative to each other in a way so that
the two limb pairs of the shield plates are aligned toward each
other while leaving one air gap each.
[0011] Based on the arrangement according to the invention and the
variant thereof, the measuring task can be solved which is to
measure a current through a U-shaped conductor unit according to
claim 8 which flows, in two parallel limbs of the U-shaped
conductor unit constituting primary conductors, in an opposite
direction. The arrangement is particularly suited to measure large
currents, for instance in a current measuring range of 2000 A while
the measurement is not invalidated by magnetic interference fields
which would otherwise come up in case of high primary currents.
[0012] It is, however, also possible that, according to claim 9, a
second measuring task may be solved wherein a difference of two
currents in two primary conductors has to be measured which
constitute an outgoing (to) conductor and a return (fro) conductor
of an electric consuming device in order to detect possible fault
currents in the load, a consuming device or a set-up. In the case
of the last-mentioned measuring task, shielding of external
magnetic interferences is of particular significance since the
magnetic field gradient resulting from the primary currents in the
outgoing and return conductor is small.
[0013] In both of the above-referenced configurations of the
primary conductors, they are substantially arranged, according to
claim 3, between the limbs of one of the two shield plates
resulting in a compact structure of the total measuring arrangement
together with the differential field sensor the effective sensor
plane of which is disposed, according to claim 4, preferably
outside of the limbs of the shield plate between which the primary
conductors are arranged. In this way, a high measuring sensibility
can be obtained without substantially attenuating the magnetic
difference field, or the field gradient, respectively, covered by
the differential field sensor.
[0014] In a basic arrangement for the potential-free measurement of
currents according to claim 5, the limbs of the two shield plates
are equally long and the two shield plates are disposed
symmetrically relative to an effective sensor plane.
[0015] The effective sensor plane is that plane of the differential
field sensor in which the transformation of the difference field,
or the magnetic field gradient, respectively, into a signal voltage
takes place, that is for instance the magneto-resistive layer and
not the substrate, or the carrier, of the sensor.
[0016] While in a modified and space-optimized total arrangement
according to claim 6 the limbs of one of the shield plates each are
equally long, the limb pairs of the shield plates have a different
length so that the one shield plate has two equally long shorter
limbs while the other shield plate has two equally long longer
limbs. The primary conductors are substantially arranged between
the longer limbs of the corresponding shield plate. This means that
the air gaps are arranged non-symmetrically relative to a middle
plane disposed in the middle of the arrangement of the two shield
plates in parallel to the webs thereof. At the same time, the
effective sensor plane is displaced relative to the middle plane
referred to. The webs are preferably plane sections of the shield
plates which connect limbs bent to form right angles at them.
[0017] Both in case of the basic arrangement and in case of the
modified arrangement, the shield plates extend in the longitudinal
direction of their limbs substantially over the differential field
sensor in order to shield it against external magnetic interference
fields. The longitudinal direction of the limbs of the shield
plates extends in parallel to the direction of the current in the
primary conductors.
[0018] In that case in which the two primary conductors constitute
component parts of a U-shaped conductor unit, the shield plates
extend, for a good shielding of the differential field sensor
against external magnetic interference fields, according to claim
10, suitably in the longitudinal direction of the limbs thereof
substantially over the length of the limbs of the U-shaped
conductor unit.
[0019] Typically, the shield plates of highly variable material may
consist, according to claim 11, of mumetal or may, according to
claim 12, be electric sheets in order to practically exclude, if
the limbs of the shield plates are correctly positioned, the
formation of any magnetic field gradient by external homogeneous or
inhomogeneous magnetic fields, substantially independently from the
interference direction of the interference fields from outside onto
the two shield plates. This means also that an inhomogeneous
magnetic field originating from a conductor which is a neighbor
outside of the arrangement of the two shield plates does not have
an interfering impact on the differential field sensor within the
shield plate arrangement.
[0020] By positioning the air gaps and/or altering the dimensions
of the shield plates, as referred to above, it is possible to
readjust, or alter, the measuring sensitivity, or the measuring
range, respectively, of the arrangement for the potential-free
measurement of currents. An additional, particularly practicable
possibility for the adjustment of the measuring range consists in
that, according to claim 13, at least one soft-magnetic insert part
shaped as an insert pin or insert strip is disposed within the two
shield plates in order to influence the magnetic difference field
in the effective sensor plane.
[0021] To this end, the at least one insert part may be arranged,
according to claim 14, between the effective sensor plane and the
primary conductors. In particular, two primary conductors which are
arranged outside of the magnetic field sensitive range of the
differential field sensor may attenuate the magnetic field decisive
for the current measurement as compared to an arrangement without
insert parts. If, however, at least one insert part is arranged in
the magnetic field sensitive range of the differential field
sensor, between the latter and the two primary conductors, the
measuring sensitivity will be enhanced. In this way, a basic
arrangement comprising the differential field sensor and the
primary conductors within the shield plates may be adapted, by
additional insert parts which are simple to handle to desired
different measuring ranges, for instance for measuring ranges from
50 to 2000 A. In addition to the positioning of the insert part, or
the insert parts, respectively, the position of the air gaps
should, as a rule, be modified. To this end, the air gaps can,
according to claim 18, be adjusted.
[0022] Typical variants of the positioning of the insert parts are
described in claims 15 through 17.
[0023] In the case of an arrangement of at least one insert part
either between the effective sensor plane and the primary
conductors or between the effective sensor plane and the nearest
neighboring shield plate, the impact of the at least one insert
part on the measuring sensitivity, or the measuring range,
respectively, is in general most significant.
[0024] By means of the at least one insert part, if necessary in
connection with an adjustment of the air gap and with the basic
set-up of the arrangement of the shield plates, of the primary
conductors and of the differential field sensor remaining as it is,
it is possible to establish in the effective sensor plane a
measuring range of for instance between 50 through 2000 A.
[0025] Exemplified embodiments of the arrangement for the potential
free measurement of currents will be explained in the following
based on five figures from which further details, particularly as
concerns the spatial-geometric disposal of the components of the
arrangement, will be obtained.
[0026] FIG. 1 is a basic arrangement in a cross section,
[0027] FIG. 2 is a modified arrangement, also in a cross
section,
[0028] FIG. 3 is a second modified arrangement including insert
parts, also in a cross section,
[0029] FIG. 4 is a third modified arrangement including one insert
part, in a cross section, and
[0030] FIG. 5 is a fourth modified arrangement without any insert
part, in a cross section.
[0031] In the basis arrangement according to FIG. 1, a first shield
plate 1 and a second shield plate 2 made of highly permeable
material are bent in an approximate U-shape. Subsequently, from an
approximately plane web 3 of the first shield plate 1, there extend
two limbs 4, 5, and from a substantially plane web 6 of the second
shield plate 2, there extend two limbs 7 and 8. The two shield
plates 1, 2 are arranged relative to each other so that their limbs
4, 5, and 7, 8, respectively, are directed toward each other, i.e.
are in alignment with one another, while one air gap 9 and 10,
respectively, is left free. The disposal of the two shield plates
1, 2 is symmetrical relative to an effective sensor plane 11, in
which there is an effective layer of a differential field sensor
12. The differential field sensor 12 may be secured,
potential-free, by an insulating holder, not shown, relative to two
primary conductors 13, 14.
[0032] The two primary conductors 13, 14, as shown in detail in
FIG. 1, may be the limbs of a substantially U-shaped conductor unit
which extend in the longitudinal direction (that is at right angles
to the plane of the drawing) in parallel relative to the shield
plates 2, 3 and the effective sensor plane 11. The extension of
these limbs in the longitudinal direction and the expansion of the
limb 4, 5, 7, 8 of the shield plates 1, 2 in the longitudinal
direction as well are in this case substantially identical. The
limbs, or primary conductors, respectively, 13, 14 are therefore
encompassed between the shield plates 1, 2. The differential field
sensor extends within this arrangement in the longitudinal
direction at the maximum as far as do the limbs 4, 5, 7, 8 of the
shield plates 1, 2.
[0033] In the case of this symmetrical arrangement of the shield
plates 1, 2, the space between the limbs 7, 8 of the lower shield
plate 2 is utilized by the integration of the primary conductors
13, 14 between the limbs while the inner space of the upper shield
plate 1 between the limbs 4 and 5 remains free, that is, it is not
used for the integration of components of the arrangement.
[0034] A first modified and in this way space-optimized arrangement
is shown in FIG. 2. It differs from the symmetrical arrangement
according to FIG. 1 in that a first shield plate 15 and a second
shield plate 16 are differently designed so that limbs 17, 18 of
the first shield plate while identical relative to each other are
shorter than limbs 19, 20 of the second shield plate 16, while the
length of the two limbs 19, 20 is identical. By the aligned
arrangement of the limbs 17, 18 with the limbs 19, 20 which are
basically positioned as in the basic arrangement according to FIG.
1, this has the result that air gaps 21, 22 between the limbs 17,
19, and 18, 20, respectively, are relatively higher in the present
arrangement than in the arrangement according to FIG. 1. Similarly,
the effective sensor plane 23 of the differential field sensor 24
is displaced upwardly.
[0035] Both in the embodiment according to FIG. 1 and in the one
according to FIG. 2, the sensor plane is disposed outside of the
limbs 7, 8, or 19, 20, respectively of the second, lower, shield
plate 2, or 16, respectively. The limbs 19, 20, again, encompass
two primary conductors which may extend beyond these limbs.
[0036] The two primary conductors 25, 26, again, may be realized in
the shape of a substantially U-shaped conductor unit.
[0037] In principle, however, the two primary conductors may also
in all the embodiments of the arrangement displayed be separate
outgoing and return conductors of an electric consuming device.
[0038] In the embodiments according to FIGS. 3 and 4, examples are
shown of the positioning of soft-magnetic insert parts in the
spaces, enclosed by two shield plates 27, 28 in FIGS. 3, and 29, 30
in FIG. 4, respectively, of arrangements for the potential-free
measurement of currents. The insert parts are designated by
numerals 31-33. They extend in the longitudinal direction
perpendicularly to the plane of the drawing, substantially over the
same length as the shield plates 27-30.
[0039] More in detail, there is arranged, in the embodiment
according to FIG. 3, an insert part 31, or 32, respectively, each
between one of the two primary conductors 34, 35 and the effective
sensor plane 36 of a differential field sensor 37.
[0040] Contrary thereto, in the embodiment according to FIG. 4,
only the one insert part 33 is positioned symmetrically to a middle
plane 38 vertical in FIG. 4.
[0041] In the two embodiments according to FIGS. 3 and 4, the
measuring sensitivity, or the measuring range, respectively, may be
adjusted by the insert parts 31-33, and regularly also by the
position of the air gaps 39, 40 and 41, 42, respectively.
[0042] The embodiment according to FIG. 3 leads to an attenuation
of the magnetic differential field, or the field gradient,
respectively, in the effective sensor plane 36, while the
embodiment according to FIG. 4, leads to a gain of the magnetic
differential field, or the field gradient, respectively, so that
the measuring sensitivity will be enhanced and the measuring range
becomes smaller.
[0043] The insert parts may in particular consist of mumetal.
[0044] The fourth modified, simplified, arrangement shown in FIG.
5, is like for instance the first modified arrangement,
unsymmetrical relative to an horizontal plane, not shown in the
drawing, particularly the effective sensor plane and actually still
further unsymmetrical than the modified arrangement according to
FIG. 2 since only the second shield plate 16' is provided with two
limbs 19', 20'. As compared thereto, the first shield plate 15' is
designed for easy production only in plane shape, quasi as a web
without a limb. Air gaps 21', 22' are provided in this arrangement
between one of the limbs 19', 20', each, of the second shield plate
16', on one hand, and the first shield plate 15', on the other,
that is, they are provided at the free ends of the limbs 19',
20'.
[0045] The limbs 19', 20' in this arrangement, too, encompass two
primary conductors 25', 26' parallel relative to each other in such
a way that above the primary conductors 25', 26', space is left for
the differential field sensor 24'. The effective sensor plane, not
shown in FIG. 5, is in this case disposed, protected, deeper than
the ends of the limbs 19', 20'.
[0046] As concerns the adjustment of the measuring range described
above, the embodiment according to FIG. 5 can be provided with one
insert part or with two insert parts.
LIST OF REFERENCE NUMERALS
[0047] 1 1.sup.st Shield plate [0048] 2 2.sup.nd Shield plate
[0049] 3 Web [0050] 4 Limb [0051] 5 Limb [0052] 6 Web [0053] 7 Limb
[0054] 8 Limb [0055] 9 Air gap [0056] 10 Air gap [0057] 11
Effective sensor plane [0058] 12 Differential field sensor [0059]
13 Primary conductor [0060] 14 Primary conductor [0061] 15, 15'
1.sup.st Shield plate [0062] 16, 16' 2.sup.nd Shield plate [0063]
17 Limb [0064] 18 Limb [0065] 19, 19' Limb [0066] 20, 20' Limb
[0067] 21, 21' Air gap [0068] 22, 22' Air gap [0069] 23 Effective
sensor plane [0070] 24, 24' Differential field sensor [0071] 25,
25' Primary conductor [0072] 26, 26' Primary conductor [0073] 27
1.sup.st Shield plate [0074] 28 2.sup.nd Shield plate [0075] 29
1.sup.st Shield plate [0076] 30 2.sup.nd Shield plate [0077] 31
Insert part [0078] 32 Insert part [0079] 33 Insert part [0080] 34
Primary conductor [0081] 35 Primary conductor [0082] 36 Effective
sensor plane [0083] 37 Differential field sensor [0084] 38 Vertical
middle plane [0085] 39 Air gap [0086] 40 Air gap [0087] 41 Air gap
[0088] 42 Air gap
* * * * *